Variable displacement pumps for hydraulic transmission controls for automatic transmissions are known.
There exist, for example, control circuits in which initially only the primary pressure control valve is active and, upon reaching the set primary pressure, the connection to the secondary hydraulic circuit opens and then the pressure acting in the secondary hydraulic circuit is active on the pump controller so that the pump control is adjusted in the direction of a swiveling-back or downward-regulating pump, i.e. the conveying volume becomes smaller. As a result, what is known as a cascade control or arrangement in series of two controller systems, namely of the primary pressure control valve and of the pump controller which can have an influence counter to one another in their control movements or control characteristics, is created, wherein, due to the downstream secondary pressure action, the pump control becomes active with a time delay after the initial adjustment of the primary pressure by the primary pressure control valve (FIG. 1.1).
Control circuits with a variable displacement pump and an automatic transmission in which a separate control pressure from a control circuit becomes active simultaneously on the pump controller of the variable displacement pump, the primary pressure control valve and, where applicable, a pressure reduction valve in the secondary or lubrication circuit are furthermore known in the prior art. This means that, depending on adjustment, possibly three difference controllers, which are all activated by the same pressure signal variable, intervene simultaneously and thus, depending on their behavior over time, mutually influence one another and can lead to instabilities (FIG. 1.2).